1. Field of the Invention
This invention relates to three phase regulated supplies utilizing switching elements to maintain output voltage regulation and more particularly to circuitry for detecting excess output ripple and regulating the supply output voltage in accordance therewith.
2. Description of the Prior Art
Three phase regulated supplies utilizing switching elements such as thyristors or triacs to maintain output voltage regulation are known. In thyristor controlled supplies the thyristor is used both as a rectifier and as a control element. For low and medium power applications the thyristors are typically placed in the secondary of a three phase transformer configuration. There are two thyristors for each phase. The output voltage of the supply is controlled by firing the thyristors in a predetermined sequence. The point at which the thyristors are fired in relation to the half cycle of ac input voltage determines the output voltage.
In triac controlled (also known as controlled ferroresonant) regulated supplies, the triac is used to switch an inductor in and out of a tank circuit electronically to thereby provide output voltage regulation. As the triac exhibits symmetrical bidirectional characteristics it functions as a bidirectional thyristor and thus only one triac is used per phase. By turning on the triac at varying phase angles with respect to the input voltage the supply output voltage is made lower or higher and thereby controlled.
Three phase regulated supplies have found application in both power supplies and in telephone central offices as battery chargers. Three phase battery chargers typically regulate their output voltage by controlling the switching element firing time in accordance with either the regulated supply output voltage and/or regulated supply output current. The regulated supply output voltage and output current are compared to known reference signals to thereby generate signals to control the firing of the switching elements. Typically the regulated supply operates under output voltage control. However if the regulated supply output current should exceed the reference setting the supply will then operate under output current control.
One problem associated with three phase regulated supplies is the excessive ripple in the output current which results if one of the input phases is either totally lost or becomes unbalanced with respect to the other phases. Loss of a phase or unbalance in the phases causes the output ripple current to increase. Where output filter capacitors are used in the three phase rectifier, the excessive ripple current can cause either failure of the filter capacitors or blowing of the associated fuses.
In the past it has been common practice to monitor the ac input voltage and if a phase was lost or an unbalance occurred, then the regulated supply was shut off to avoid any damaging affects from excessive ripple current. However in many instances such a drastic remedy is not necessary. Even in those circumstances where a phase is lost or a phase unbalance occurs the supply is still capable of generating some useful output voltage. Very rarely does the supply ever operate at full load capacity. Such a situation might, for example, occur where following a total power failure at a telephone central office it is necessary that the regulated supply both recharge the batteries and supply the load. Such conditions occur infrequently. Thus in most circumstances it is desirable to keep the regulated supply operating and supplying a load even in the presence of either a lost input phase or an input phase unbalance.
The circuit of the present invention allows the three phase regulated supply to continue to operate and supply light load conditions under the above circumstances, as long as the output ripple does not exceed a predetermined magnitude. Thus, in addition to controlling the firing times of the switching elements as a function of the regulated supply output voltage and/or output current the circuit of the present invention allows the firing times of the switching elements to be controlled as a function of the magnitude of the ripple present at the supply output.